There are a number of commercially available products which provide sensing, control, and communications in a network environment. These products range from elaborate systems having a large amount of intelligence to simple systems having little intelligence. By way of example, such a system may provide control between a light switch and a light. When the light switch is operated, a digital code pattern is transmitted by one cell over power lines or free space and is received by another cell at the light. When the code is received, it is interpreted and subsequently used to control the light. Such a system, comprising a network of intelligent cells in which the cells communicate, control and sense information, is described in U.S. patent entitled, "Network and Intelligent Cell for Providing Sensing, Bidirectional Communications and Control", U.S. Pat. No. 4,969,147, issued Nov. 6, 1990, which application is assigned to the assignee of the present invention.
The transmitting and receiving of digital data can be performed by a series of transceivers, each of which is connected to an individual cell of a network. These transceivers may communicate with one another in numerous different ways over various media and at many different baud rates. They may, for example, transmit and receive radio frequency (RF) or microwave frequency signals through antennas. The transceivers could be connected to standard communications lines, such as twisted pair lines, fiber optic cables, and coaxial cables. Alternatively, infrared or ultrasonic implementations can be utilized to effectuate the transmission of data between the transceivers. Indeed, even power lines have been employed as a transmission medium by implementing spread spectrum techniques.
Selecting a medium involves a trade-off between cost and performance. Although fiber optic and coaxial cables meet stringent transmission criteria, these media are relatively expensive. Moreover, significant costs are incurred in order to physically route these cables between each of the transceivers.
It is often more cost effective to take advantage of an already existing medium, such as 120 volt AC power distribution lines, over the airwaves as RF, or twisted pair lines found in telephone communications. However, both power lines and RF transmissions are highly susceptible to noise and/or other electrical interferences. Hence, digital signals often suffer great degradation. Twisted pair lines, which are normally used to conduct analog voice signals for telephone systems can, instead, be utilized in conducting digital signals. Although twisted pair lines generally exhibit better transmission characteristics, they are nonetheless susceptible to signal distortions and attenuation. The distortions and attenuation tend to increase over longer distances and higher data rates. The distortion and attenuation problems are attributable in part to the transmission of digital data as a series of square waves. The edges of the square waves involve fast transitions in voltage. In turn, these fast transitions contain high frequency harmonics, which result in reflections, ringing, etc. Ideally, the transmitted waveform should resemble a sine wave having one fundamental frequency. Thereby, the single frequency minimizes the harmonics being propagated through the twisted pair lines and the attendant reflections, ringing, etc.
Converting the square wave to a sinusoidal waveform is a rather complicated task. It can be approximated by implementing a complex analog filter. The complexity is magnified in order to accommodate different data rates. A programmable filter design implementation is a costly and complex solution. The transmitted sinusoidal waveform often is degraded by noise and media degradation, and the receiver is needed to recover the original sinusoidal waveform in the presence of this noise.
Thus, there is a need in the prior art for transceivers to minimize the harmonic content of digital data being sent over twisted pair lines in an effort to reduce distortions caused by reflections, ringing, etc. It would be highly preferable for such a transceiver to be simple, efficient, and cost-effective.